To study the thyroid growth and the production and properties of transplantable thyroid tumors in the rat: changes occurring during growth, hyperplasia and involution, histologic types of tumors and their progression, and the correlation of histology with the biochemistry and physiology of the tumors. We are also studying a variety of types of cells just recently recognized to be present in the normal thyroid gland and which may be the cells of origin of some of the different histologic types of thyroid tumors which have been observed in the past.